Because sailboats are subject to the caprice of the wind, they are less maneuverable than other boats. In particular, maneuverability is impaired by the awkwardness of coming about (turning from one diagonal upwind course, or tack, to the other). To come about, the sailor typically must: (1) push the tiller hard to lee, (2) switch the jib (if there is one), (3) duck the boom and (4) leap to the other side, all the while holding the tiller and mainsheet and keeping track of nearby boats or other obstacles. If the boat has too little momentum to coast completely around the turn, it ends up pointed into the wind with sails flapping in a position called “caught-in-irons” from which escape is awkward. Other, less drastic turns are also often difficult to execute quickly and accurately, especially in strong winds with the boat heeling (tipping away from the wind) and the crew hiked out (with their weight out to windward). Moreover, conventional sailboats cannot readily slow down or stop in a well-controlled way. This often makes landing at a dock or mooring under sail a difficult and uncertain maneuver.
An unusual type of sailboat (the proa), which has longitudinal but not lateral symmetry, solves some of these problems but poses others. The proa has a double-ended hull with an outrigger to windward on which the crew sits. The weight of the crew balances the heeling moment of the wind force on the sail. Like conventional sailboats, proas can tack upwind, but they come about in a different way. Instead of turning into the wind, they turn away from the wind, so that at mid-turn the wind is on the beam (perpendicular to the axis of the boat). At this point, the crew turns the sail to catch the wind on the other side, causing the proa to reverse direction. It then turns upwind on the new tack. Coming about in this way is called shunting. Shunting does not depend on momentum and thus avoids the risk of getting caught-in-irons. Proas can also come to a controlled stop with the wind on the beam, and then move off in either direction.
Offsetting these advantages of the proa is awkwardness in making downwind turns. A conventional sailboat moving before the wind can turn directly either way, provided care is taken to control a possible jibe, in which the boom swings across the cockpit. A proa can make the corresponding turn only by executing a complete shunt. If it turns directly, the outrigger can end up on the wrong side where, instead of counter-balancing heeling, it reinforces it.
Some sailboats of the prior art have elements that in some ways are similar to elements of the boat described here, but no combination of elements was found in the prior art that yields solutions to all of the maneuverability problems described above.